The present invention relates to a method of thermal analysis for determining an appropriate heating condition for heating an object in accordance with a required temperature profile. The present invention also relates to an apparatus of thermal analysis, a heat controller and a heating furnace using such a method. More specifically, the present invention relates to a method of thermal analysis, and a reflow furnace using such a method for determining a proper heating condition for heating a circuit substrate. Electronic components are mounted on the circuit substrate via cream solder which is melted when heated. After the circuit substrate is cooled, the melted cream solder solidifies, thereby welding the electronic components onto the circuit substrate. The present invention also relates to a program and a computer readable recording medium recording such a program, which may be used for making a computer process the method of thermal analysis.
When heating an object in a heating furnace, it is necessary to control heating temperature and heating time in a predetermined manner so as to keep the object at a proper temperature for a certain period of time and not to overheat beyond the upper limit temperature of the object. Prudent thermal analysis and temperature control is critical for not only keeping the object at such proper temperature for a certain period of time in a heating furnace, but also for heating the object in accordance with a required temperature profile during preheating, main heating and cooling stages.
The following description is made by referring to a reflow furnace as an example, which is used for soldering electronic components onto a circuit substrate such as an electronic circuit board (herein after referred to as a “circuit board”). In a reflow process, at first, cream solder is printed on a circuit board, and electronic components are mounted on the circuit board at corresponding predetermined positions. The circuit board is then introduced into a reflow furnace for heating and melting the solder, thereby soldering and securing the components onto the circuit board. In order to avoid any heat destruction of the object (i.e., electronic components as well as the circuit board) due to steep temperature increase in the reflow furnace, the object is first heated at a relatively low temperature at a preheat stage. Such heating at the preheat stage is also preferable to activate fluxes contained in the cream solder, such as anti-oxidization flux, for improving soldering quality. The object is then heated at a reflow stage, where the object is kept at a temperature over a melting point of the solder for a predetermined period of time so that the solder may be completely melted. After the reflow stage, the object is cooled for solidifying the solder to secure the electronic components onto the circuit board.
In view of a recent environmental conservation demand, a trend has existed for some time that conventional solder materials made from tin-lead compound are being replaced by lead-free materials, such as tin-zinc-bismuth compound, which do not contain any poisonous materials. The melting point of such lead-free solders is generally somewhere around 220° C., which is higher than the melting point of about 190° C. for lead-based solders. Therefore, the lead-free materials should be heated at a higher temperature than the conventional solder during the reflow heating operation for complete melting. On the other hand, in order to prevent any heat destruction of the electronic components and the circuit board during such heating, the object, or the circuit board having electronic components thereon, should not be heated over the upper limit temperature, which is a temperature at which all the components and the circuit board may endure and sustain their intended functions. For example, in case one of the electronic components to be mounted on the circuit board is an aluminum electrolytic condenser, such upper limit temperature is about 240° C. This means that when the heating temperature for heating the object is too low (e.g., below 220° C.), the electronic components may not be securely soldered onto the circuit board, while on the other hand, when the heating temperature is too high (e.g., over 240° C.), the electronic component may be damaged. Consequently, as described above, severe temperature control for heating the solder at a temperature over its melting point, and yet at lower temperature than the upper limit temperature of the respective components is needed for achieving reliable soldering operation in the reflow furnace. Toward this end, heating conditions including the temperature of a heating source, such as heat blower or a heat panel, and transfer speed for moving the object through the heating furnace should be properly determined in accordance with a temperature profile corresponding to required heating conditions for heating the respective object.
There are two types of heating methods applicable to heating furnaces. One of the methods is convection type heating in which heated air from a heat source such as electricity or burning gas or oil is blown toward the object, and another method is radiation type heating in which a heat source such as an infrared radiation heat source radiates heat toward the object. There is a variety of heating equipment, such as a reflow furnace, a heat treatment furnace, a sintering furnace, a baking oven such as that used for making ceramics, a melting furnace, or incinerating equipment. Depending on a purpose of heating and/or a kind of heating equipment, an appropriate heating type may be selected. In case when severe temperature control is required, such as for a reflow furnace for soldering electronic components onto a circuit board, convection type heating is typically selected because of its relatively easy temperature controlling capability.
In a conventional way of determining reflow heating conditions, at least one thermocouple is fixed to the circuit board, and temperature change at such a fixed point is measured during heating. Such measurement is repeated by changing a heating condition of the reflow furnace one after another until an appropriate heating condition is identified. Each time of changing the heating condition, a relatively long period of time is required for waiting for the temperature of the furnace to become in a stable condition for the next trial. Typically, such repetition is required for about ten times until the appropriate heating condition is determined. In addition to such lengthy time for waiting, inspiration and experience of a skilled operator is inevitable for setting a subsequent heating condition based on preceding measurement results. Moreover, even if an appropriate heating condition is determined through such trial and error efforts, it is sill not certain as to whether such a heating condition is optimum or not, namely, whether such a heating condition may easily meet the required conditions, or barely meet the conditions.
In the prior art, some alternative methods of determining reflow heating conditions have been proposed for avoiding such a laborious method with lengthy operations conducted by a skilled operator. Japanese patent application laid open to public No. 45961/2002-A discloses a method for determining an optimum heating condition, including steps of:
heating a test sample with known physical characteristics in a heating furnace, and measuring a temperature change thereof;
processing the temperature change with a differential equation by using a heating feature of the heating furnace as a parameter; and
repeating such processing by changing a value representing the heating feature of the heating furnace until a difference between the measured value and the processed value becomes minimum.
Japanese patent application laid open to public No. 201947/1999-A (U.S. Pat. No. 3,274,095) discloses a method of controlling a heat source including steps of:
setting a heating condition for each of a plurality of heating sources to be used for heating an object;
heating the object and detecting temperatures of a plurality of detecting points of the object;
calculating a relationship between a difference of the heating condition for each heat source and a difference of the detected temperature of each detecting point of the object, and
based on the result of the calculation, determining a heating condition for each heating source that may make the temperature of the object to be the same as the targeted temperature.
Both of these methods, however, require physical characteristics of the object (or a test piece) in order for determining an optimum heating feature or controlling the heating source. Accordingly, it is necessary to obtain individual physical characteristic data of the object, and input these data beforehand. Especially in these days, one circuit board typically has about 100 electronic components to be mounted thereon. Design changes and component combination changes occur very often. In view of these circumstances, it is rather difficult at an operation sight to implement such complicated and time consuming methods which require obtaining physical characteristics for individual measuring points, or electronic components, of the object. In some cases, such as the case when the object is formed by mixed components or a combination of many components, obtaining physical characteristics of those components is difficult.
U.S. Pat. No. 6,283,378 discloses a method of adjusting a boundary condition temperature of a heating furnace having a plurality of heating sections, including steps of:
measuring both of a boundary condition temperature and a blowing heat temperature for each of the heating sections, and
adjusting the boundary condition temperature by an amount equivalent to a minimum difference between the boundary condition temperature and the blowing heat temperature among the differences for all of the heating sections. According to this method, however, adjustment is made only by parallel translation of a temperature profile, which is to move a temperature profile based on a single factor without considering respective differences at each of the boundaries of the heating sections. Therefore, it is difficult to perform an accurate simulation, especially when a peak temperature of the object does not exist at said boundary, or when the temperature profile is formed by complicated curves. Moreover, since an adjustment of the heating furnace as a whole is made by a single temperature control, there exists a problem that specific heating conditions at each of the plurality of measuring points are neglected.
Accordingly, in view of the above mentioned problems of the conventional methods, the purpose of the present invention is to provide a method and an apparatus of thermal analysis as well as a heating furnace which may be used for determining a proper heating condition of a heating furnace in an effective manner, without requiring physical characteristics of the object to be heated, or without conducting repetitive heating and measuring processes of a sample object in a trial and error manner.
It is also a purpose of the present invention to provide a heat controller capable of implementing the above mentioned method, computer readable recording medium which can be used for the heat controller, and a program to be recorded in such a recording medium.